Didact (previously BtnQ)

A tiny macOS menu-bar app to control an external monitor over DDC/CI — no settings window, no ~1 GB control panel, just standard macOS menus.

Initially built for the BenQ RD280UG (which it ships with a profile for), Didact now works toward supporting any DDC/CI monitor: a built-in wizard detects the standard controls and learns the rest, and every monitor-specific detail lives in a shareable JSON profile — so new monitors need no code changes.

Didact vs BenQ Display Pilot 2

	Didact	Display Pilot 2
Download	338 KB	404 MB
Installed	360 KB	936 MB

…about 1,200× smaller to download and over 2,600× smaller installed, for the controls you actually use.

Requirements

• Apple Silicon Mac (DDC is done via IOAVService, Apple-Silicon only).
• A monitor connected over DisplayPort / USB-C / HDMI with DDC/CI enabled.
• macOS 13.0 or later.

How it works

• DDC transport is a vendored copy of AppleSiliconDDC (MIT). The private CoreDisplay/IOKit symbols it needs are declared in AppleSiliconDDCBridge.swift via @_silgen_name, so the whole library is just two Swift files — no bridging header, no SPM dependency. CoreDisplay.framework is linked via OTHER_LDFLAGS = -framework CoreDisplay.
• The app is not sandboxed (ENABLE_APP_SANDBOX = NO): DDC needs raw IOKit access, which the App Sandbox forbids.
• The DDC/VCP map was ported from bebenqli, a Linux TUI for the same panel.

Adding support for another monitor

Drop a JSON file into either:

• the app bundle (Didact/Monitors/), or
• ~/Library/Application Support/Didact/Monitors/ (no rebuild — use Didact ▸ Open Monitors Folder, then Reload Configs).

Config format

{
  "name": "BenQ RD280UG",          // shown in the menu
  "match": ["RD280U", "RD280UG"],  // case-insensitive substrings of the display's product name
  "controls": [
    { "kind": "group",   "label": "Image" },
    { "kind": "section", "label": "Night Protection" },

    // Slider. min/max/step are decimal.
    { "kind": "range", "label": "Brightness", "vcp": "10", "min": 0, "max": 100 },

    // Pick-one. Each option value mirrors the DDC docs (hex string) or a decimal number.
    { "kind": "cycle", "label": "Source", "vcp": "60",
      "options": [
        { "value": "0x0f", "label": "DisplayPort" },
        { "value": "0x11", "label": "HDMI" },
        { "value": "0x13", "label": "USB-C" }
      ] },

    // On/off.
    { "kind": "toggle", "label": "Auto Brightness", "vcp": "e2", "onValue": 255, "offValue": 0 }
  ]
}

Field reference

Field	Applies to	Meaning
kind	all	group, section, range, cycle, or toggle
label	all	Menu text
vcp	range/cycle/toggle	VCP feature code, hexadecimal (e.g. "60", "d9")
min/max/step	range	Slider bounds (decimal); step defaults to 1
options	cycle	List of { value, label }
onValue/offValue	toggle	Values written for on/off
channel	range/cycle/toggle	High byte written for 16-bit multiplexed registers (e.g. Moon Halo on d9)
readChannels	range/cycle/toggle	High byte(s) that identify this control's value on read (a multiplexed read returns only the last-touched channel). Defaults to channel.
noRead	any control	Value can't be read back; Didact remembers the last value you set
noVerify	any control	Monitor reports a bogus value after a write

Values: a JSON number is decimal; a JSON string is hexadecimal ("0x30" or "30"). vcp is always hex.

Multiplexed registers: some BenQ features share one VCP code, selected by the high byte. Moon Halo brightness and colour temperature both live on d9 (channel: "0x01" and channel: "0x07"); Didact writes (channel << 8) | value.

Debugging — dump DDC values

Tools/dump.sh compiles a small CLI from the app's own DDC code and prints the current value of every candidate VCP code, decoded against the config:

./Tools/dump.sh                       # uses Didact/Monitors/BenQ-RD280UG.json
./Tools/dump.sh path/to/other.json    # decode against another config

Thanks

Didact is built directly on these two projects:

• AppleSiliconDDC by @waydabber (MIT) — the DDC/CI transport, vendored into Didact.
• bebenqli by @iurev (MIT) — the BenQ RD280UG DDC/VCP map and the baseline-sweep discovery behind Listen mode.

License

Didact is released under the MIT License. Both vendored/ported components above are also MIT-licensed; their notices are reproduced in the LICENSE file.